Cutting inserts containing superhard crystals, such as diamond, cubic boron nitride, .alpha. Si.sub.3 N.sub.4 or .beta. Si.sub.3 N.sub.4, exhibit excellent wear-resistance characteristics.
An increasing number of techniques have been developed for employing .alpha. or .beta. Si.sub.3 N.sub.4 in such cutting inserts. Representative of such techniques are those disclosed in U.S. Pat. Nos. 4,264,548; 4,264,550; and 4,280,973. None of these techniques relate to cutting inserts including a dispersion of diamond crystals.
However, high speed machining of Al-Si alloys requires the use of diamond crystals in cutting inserts, since Al-Si alloys, particularly alloy 390, is very abrasive.
Recently, an economical and rapid technique for forming cutting inserts having a high concentration of diamond crystals at the cutting edges, has been devised, and is described in U.S. patent application Ser. Nos. 167,019 and 167,196, by Dr. John M. Ohno, both filed July 9, 1980, and assigned to the assignee of the present invention, now abandonded, and in their respective co-pending continuation application Ser. Nos. 313,241, now U.S. Pat. No. 4,428,755 and 312,987, now U.S. Pat. No. 4,417,906, filed Oct. 20, 1981, the entire disclosures of which are hereby incorporated by reference. Disclosed therein is a straightforward technique (hereinafter referred to as the "press and treat" technique) for economicaly and rapidly forming a composite body for use as a cutting insert. Very briefly, the press and treat technique involves the preparation of a first or a crystal dispersion of super-hard crystals such as diamond or cubic boron nitride crystals in carbon black and a second or core dispersion of carbon black, carbon fiber and filler material such as .beta. or .alpha.-silicon carbide. The two dispersions are individually mixed with a small amount of temporary binder, such as paraffin, to lend a sufficient green strength to the two dispersions upon cold compaction thereof. After compacting the two dispersions together in a desired configuration, the compact is vacuum heated in the presence of silicon to burn off the paraffin and to allow the silicon to infiltrate both dispersions. Upon further heating, and without the need for the constant application of any type of pressure to the insert, the silicon reacts with the carbon black to form a .beta.-silicon carbide and silicon matrix which bonds both dispersions both internally and to each other.
In general, the machining of Al-Si alloy, particularly alloy 390, is extremely difficult, not only due to the highly abrasive nature of the alloy, but also due to the typically rough and interrupted cutting encountered with Al-Si 390 castings, which typically include a number of ingate and inclusions on the machined surface. Typical operating requirements when machining alloy 390 are given below in Table I for finish machining and in Table II for rough machining.
TABLE I ______________________________________ SPEED DEPTH OF CUT FEED/REV. ______________________________________ 3000-2000 SFPM 0.02-0.05" .0025-.0027" ______________________________________
TABLE II ______________________________________ SPEED DEPTH OF CUT FEED/REV. OPERATION ______________________________________ 1200 SFPM .075" .0100" O.D. Turning 1400 SFPM .060-.080" .0101" Facing ______________________________________
In order to cope with rough machining conditions, several changes to the basic cutting insert as disclosed in the above-mentioned co-pending patent applications are desirable. One such modification is to provide a cutting insert used for the rough machining of Al-Si alloy with a bi-level top surface to reduce the bending moment imparted to the insert during the machining operation, as disclosed in co-pending U.S. patent application Ser. No. 331,365, filed concurrently herewith, by Dr. John M. Ohno and assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated by reference.
Another technique for coping with the rough machining operation is to produce a cutting insert composite having a particular configuration which employs a prepressed centerpiece or pellet, as described in co-pending U.S. patent application Ser. Nos. 331,370 and 331,376, filed concurrently herewith, by Dr. John M. Ohno and assigned to the assignee of the present invention, the entire disclosures of which are hereby incorporated by reference.
Although the above-mentioned techniques improve the capabilities of the cutting inserts to provide rough machining of Al-Si alloy, buildup of the Al-Si alloy, although significantly reduced compared to prior art inserts, still exists.
More specifically, in producing such inserts, it is necessary to provide free silicon in excess of that required to react with the carbon black in the composite to form .beta.-silicon carbide, since the lack of such free silicon in the composite may contribute to voids in the micro structure as illustrated in FIG. 1. Thus, during the step of silicon infiltration, silicon in excess of that required for reaction with carbon black is provided to thereby fill the voids, the composite being bound by a matrix of .beta.-silicon carbide and silicon. The complete infiltration of elemental silicon, including the excess free silicon discussed above, produces a composite having substantially no voids, as illustrated in FIG. 2. A specific example of the composite produced in accordance with the press and treat technique having an excess of elemental silicon is described in Table III prior to silicon infiltration and in Table IV after silicon infiltration.
TABLE III __________________________________________________________________________ DIAMOND CARBON BLACK PARAFFIN __________________________________________________________________________ Cutting Edges 92-93 2-3 5 __________________________________________________________________________ SiC CRYSTAL C.B. + C. FIBER PARAFFIN BORON __________________________________________________________________________ Main Body 75-79 13-18 5 1-1.5 __________________________________________________________________________
TABLE IV ______________________________________ FREE Si BY WEIGHT (%) BY VOLUME ______________________________________ Cutting Edges 7-10 10-13 Main Body 14-17 18-21 ______________________________________
The elemental silicon on the surface of the composite reacts with the Al-Si work material causing a slight buildup of the Al-Si alloy on the cutting edge.
FIG. 3 illustrates a tungsten carbide insert containing 6% cobalt after machining an Al-Si (16-18%) alloy for only two minutes. The adhesion of the Al-Si material to the insert due to the reaction of Al-Si and Co is highly detrimental to the cutting insert.
FIG. 4 illustrates Al-Si buildup on an Al.sub.2 O.sub.3 coated tungsten carbide insert after machining the same Al-Si alloy for only five minutes. Again, the effects of the metal buildup are readily evident.
FIGS. 5a and 5b illustrate an indexable diamond composite insert containing approximately 10% elemental silicon produced in accordance with the press and treat technique, after machining the same Al-Si alloy for four hours and thirty minutes. The dramatic improvements in wear and metal adhesion are readily apparent using such insert, but a further reduction in the adhesion of the Al-Si alloy to the insert is desired and would be of great benefit, since such buildup may shorten the life of the insert and reduce the quality of the surface finish.